Datasheet

I
/I
TRF3703-33
I
/I
V1
V1
R1
R1
R3
R3
V2
DAC5688
DAC5688
I
/I
TRF3703
V1
R1
R1
R3
R3
Filter
R4
V1
R2
R2
V2
DAC5688
SLLS880C –DECEMBER 2007–REVISED AUGUST 2010 www.ti.com
Figure 50. DAC5688 to TRF3703-33 Interface
In most applications a baseband filter is required between the DAC and the modulator to eliminate the DAC
images. This filter can be placed after the common-mode biasing network. For the DAC to modulator network
shown in Figure 51, R2 and the filter load R4 need to be considered into the DAC impedance. The filter has to be
designed for the source impedance created by the resistor combination of R3 // (R2+R1). The effective
impedance seen by the DAC is affected by the filter termination resistor resulting in R1 // (R2+R3 // (R4/2)).
Figure 51. DAC5688 to Modulator Interface with Filter
Factoring in R4 into the DAC load, a typical interface to the TRF3703-17 with V1 = 5V and V2 = 0V results in the
following values: R1 = 72Ω, R2 = 116Ω, R3 = 124Ω and R4 = 150Ω. This implies that the filter needs to be
designed for 75Ω input and output impedance (single-ended impedance). The common mode levels for the DAC
and modulator are maintained at 3.3V and 1.7V and the DAC load is 50Ω. The added load of the filter
termination causes the signal to be attenuated by –10.8 dB.
A filter can be implemented in a similar manner to interface with the TRF3703-33. In this case it is much simpler
to balance the loads and common mode voltages due to the absence of R2. An added benefit is that there is no
loss in this network. With V1 = 5V and V2 = 0V the network can be designed such that R1 = 115Ω, R3 = 681Ω,
and R4 = 200Ω. This results in a filter impedance of R1 // R2=100Ω, and a DAC load of R1 // R3 // (R4/2) which
is equal to 50Ω. R4 is a differential resistor and does not affect the common mode level created by R1 and R3.
The common-mode voltage is set at 3.3 V for a full-scale current of 20mA.
For more information on how to interface the DAC5688 to an analog quadrature modulator please refer to the
application reports Passive Terminations for Current Output DACs (SLAA399) and Design of Differential Filters
for High-Speed Signal Chains (SLWA053).
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